Planar RF/microwave/mm-wave antennas, whether mounted to an airframe, vehicle, helmet or radio housing, are often backed by a conducting layer. Often times the antenna assembly also requires that the antenna be conformal to the conducting surface to which it is to be mounted. In the case of biomedical sensing, and in particular for radiometric sensing, it is necessary to have a flexible, low-profile antenna that is adjustable to the environmental loading effects that arise when the antenna comes into close proximity to an object or material (such as the human body). Antenna assemblies for use in biomedical radiometric sensing applications, such core body temperature measurement and pressure ulcers progress monitoring, require a flexible, low-profile antenna capable of adjusting to the environmental loading effects experienced by the antenna.
In the case of a radiometric antenna having a conductive layer backing, the presence of the conducting layer greatly limits not only the type of antenna element that can be used, but also the extent to which the profile of the antenna assembly can be reduced. In an antenna assembly employing an antenna element and a conducting layer, the conducting layer must be separated from the antenna element by an effectively large distance due to the natural tendency of ground currents to inhibit efficient radiation of the antenna element, thereby increasing the profile of the antenna assembly.
The microstrip patch antenna is commonly known in the art for the design of planar radiating elements above a conducting layer. The microstrip patch antenna is typically narrow-band, and bandwidth enhancement requires a large antenna-to-ground separation. In a low-profile antenna, the large antenna-to-ground separation is undesirable because it increases the profile of the antenna assembly. Additionally, the designs known in the art for microstrip patch antennas of this type do not allow for end-fire radiation.
Commonly, there is a need to severely limit background radiation for highly sensitive sensing applications. The need to severely limit background radiation in these applications requires backing the printed antenna element with ground plane shielding. However, it is often also required that these antenna assemblies are low profile antenna assemblies and as such, the ground plane must be placed in close proximity to the antenna element to reduce the profile of the assembly which also results in poor radiation characteristics of the antenna due to cancellation from image currents. Moreover, in the case where multiple antennas share the same ground plane, the surface currents add unwanted mutual coupling.
It is known in the art to reduce the ground interference of the low-profile antenna assembly by introducing a textured periodic surface above the ground plane that alters electromagnetic characteristics of the ground place. This textured periodic surface is known in the art as a high impedance surface, frequency-selective surface (FSS) or electromagnetic band gap (EBG) structure and prevents the propagation of radio frequency surface currents within the band-gap structure. The limiting effect of ground plane interference has been addressed by individuals in the art through electromagnetic band-gap (EBG) technology. However, work in the art has not addressed the need for tuning of the antenna to adjust to environmental loading effects experienced by the antenna when it is placed in close proximity to an object or material.
Accordingly, what is needed in the art is a low-profile, tunable electronic-band gap antenna assembly that is also flexible and therefore suitable for conformal mounting.